HISTORY OF ZOOLOGY. 



27 



"by many transitions; in other cases sharply circumscribed groups 

 of forms, or races, can be distinguished within the same species. 

 In the race, the peculiar characteristics are inherited from genera- 



Fio. lc. English fantail pigeon. (After Darwin.) 



tion to generation with the same constancy as in good species. 

 This is shown in the human races, and in many pure, cultivated 

 races of domesticated animals. 



Physiological Characters. A. Crossing of Species and Varie- 

 ties. A critical examination leads to the conclusion that Mor- 

 phology is indeed useful for grouping animals into species and 

 varieties, but that it leaves us completely in the lurch when it is 

 Called upon to show the distinctions between what should be called 

 a species and what a variety. Therefore there remains open to 

 the systematist only one resource, i.e., to summon Physiology 

 to his aid. This has been done, and it has disclosed considerable 

 distinctions in reproduction. We should expect a priori that the 

 individuals of different species would not reproduce with each 

 other; on the other hand under normal conditions the individuals 

 of one and the same species, even though they are of different 

 varieties or races, should be entirely fertile. One must beware of 

 arguing in a circle in proof of these two propositions; it would be 

 an argument in a circle if an experimenter should regard two 

 animals as representatives of one species only because they proved 

 to be fertile together, while under their former relations he 

 assigned them to different species. Bather the question for him 

 must read: does physiological experiment lead to the same 



